Bu şablon çok sade; üstelik masaüstündeki kısayol sayesinde ona hemen ulaşıyorum.

In Turkish, the present-tense form of to be is often not expressed in simple 3rd-person sentences.

So:

Bu şablon çok sade
literally works as
This template very plain

but it naturally means:

This template is very plain/simple.

You can add -dir for a more formal, definite, or explanatory tone:

Bu şablon çok sadedir.

That is grammatical, but in everyday Turkish, the shorter version is much more common.

Üstelik means something like:

• moreover
• besides
• what is more
• on top of that

It adds an extra point that strengthens what was just said.

So the sentence structure is:

• first point: Bu şablon çok sade
• additional point: üstelik ... ona hemen ulaşıyorum

It has a slightly emphatic feel, as if the speaker is saying:

It is very simple, and what is more, I can access it immediately.